In radio communications systems, a transmitter transmits signals that are picked up and processed by a receiver at some distance from the transmitter. A few systems include only one transmitter and one or more corresponding receivers, and thus support only one-way communications. A broadcast television system can be viewed in this way, where a transmitter corresponding to a single television station sends one-way signals to multiple television receivers. Similarly, a few communication systems include only a single receiver that monitors the signals transmitted from several transmitters. Again, these systems support communications in only a single direction. However, there are many communication systems that require bi-directional communication—these systems require both a transmitter and receiver at each system endpoint, as shown in transceivers 110 and 150, in FIG. 1. Often, the transmitter and receiver for a given endpoint, such as transmitters 115 and 165 and receivers 125 and 155 in FIG. 1, are housed in the same device, and may share one or more antennas.
If both transmitters in a communications link are transmitting simultaneously, there exists the potential that either receiver is unable to receive signals, due to interference from its corresponding transmitter or from other transmitters. A transmitter is designed to transmit a large enough power signal to overcome the loss inherent in transmitting over a distance so that the signal can still be received. Conversely, a receiver is designed to be sensitive to extremely small signals to ensure that a transmitter can minimize the amount of power transmitted over a specified distance. Because of the receiver's sensitivity, it is possible for an unexpectedly large signal to interfere with the receiver operation, or even to physically damage sensitive radio components. Since the transmitter in a radio system is by necessity a high-power signal, it may easily interfere with a co-located receiver in the absence of careful system design.
Of course, strong signals that are not part of the normal operation of the communication system can also interfere with the receiver. Possible interferers include television transmitters, radar systems, electrical noise from industrial facilities, or other communication systems. There may be more than one interferer present. Thus, in addition to coordinating transmissions and reception within a given system, a well-designed system must also be designed to accommodate interference arising outside the system. Further, the system should be designed to minimize potential interference to other radio systems.
In radio transceivers, a power amplifier is used to amplify the transmit signal to an appropriate level for transmission across the air (or through a cable or waveguide). The power amplifier, like any other active device in a signal processing chain, adds noise and distortion to the signal. However, because the power amplifier signals are generally quite large, the noise and distortion introduced by the power amplifier can be particularly pronounced, especially when the power amplifier is designed to minimize power consumption. Power amplifier linearization techniques may be used to reduce the distortion introduced by the power amplifier, but this generally does not impact the lower-level broadband noise emitted by the amplifier. Furthermore, specifications for a given radio system and/or government regulations may impose stricter requirements on emissions from the power amplifier, particularly with respect to undesired out-of-band emissions, than are readily achievable through linearization techniques.
The noise and distortion emissions from the power amplifier can be viewed as an “error signal” added by the power amplifier to the desired transmit signal. Generally it is necessary to remove a large portion of this error signal before the signal is transmitted into free space via an antenna. In a conventional radio system, a transmit filter, which may be part of a radio duplexer circuit, serves the function of rejecting these error signals.
One technique that is sometimes used to “linearize” a power amplifier's response is called “pre-distortion.” With this technique, a power amplifier input signal is enhanced with another signal that effectively anticipates the distortion produced by the PA. This pre-shaping of the power amplifier's input signal to anticipate the distortion by the power amplifier can significantly reduce the effective distortion of the desired transmit signal. Both analog and digital pre-distortion techniques are possible.
One method for coordinating the transmission and reception of signals to reduce the impact of undesired emissions is to have the transmitters and receivers for a given system all operate on the same frequency, and then coordinate which radio system is receiving while the other is transmitting, and vice versa. This approach is called time-division duplex (TDD) communication. A portion of a radio transceiver 200 suitable for use in a TDD system is shown in FIG. 2. The transmitter side of radio transceiver 200 includes a power amplifier 210 and a circulator 220, while the receiver side includes a low-noise amplifier 230. Both sides are coupled to an antenna path through switch 240; the antenna path comprises filter 250, antenna cable 260, and antenna 270.
As seen in the radio transceiver 200, in a TDD system both the receiver and transmitter at a given endpoint can use the same filter pass-band to reject interfering signals picked up by the antenna 270 from outside sources, in other frequency ranges. However, other methods must be found to ensure that radio systems within the communication system do not interfere with one another. In a TDD system, the timing of radio transmission and reception is carefully managed so that a transmitter is transmitting when a receiver at the other radio system is receiving and vice versa.
Another approach to coordination is called frequency-division duplex (FDD) communication, in which transmissions are separated by frequency. Thus, the transmitter of one radio system is on one frequency (e.g., f1), and the corresponding receiver (or receivers) at the other end of the communication link is tuned to the same frequency. At the same time, the transmitter and corresponding receiver (or receivers) of the other radio system is tuned to another frequency (e.g., f2). Because the transmitter and receiver pairs are on different frequencies, filters can now be used to ensure the transceivers do not interfere with each other, as well as to reject other interfering signals. This is shown in FIG. 3, which illustrates a radio transceiver 300 capable of full, frequency-division duplexed communication. Outgoing transmissions from power amplifier 210 are separated in frequency from incoming transmissions for low-noise amplifier 230, and are separately filtered by transmit filter 315, which reduces noise in the receiver band, and receiver filter 320, which rejects emissions in the transmitter band. Together, these filters form duplexer 320 (sometimes called a diplexer).
FIG. 4 illustrates an alternative configuration for a frequency-division duplexed radio transceiver 400, in which the receiver and transmitter each has its own antenna 270. Very limited isolation between the transmitter and receiver is provided by the physical separation of the antennas 270. The transmit filter 410 and receive filter 420 provide additional isolation, as each can be tuned to reject the frequency passed by the other. FIG. 5 illustrates yet another configuration for a frequency-division duplexing transceiver 500, in which circulator 220 serves as the duplexing element. Circulator 220 provides some isolation between the transmitter and receiver components; additional isolation is provided by the transmit filter 510, which is configured to reject noise in the receiver band, and receiver filter 520, which is configured to reject transmitter band emissions.
In an FDD system, electronic cancellation circuits may replace some or all of the transmit and receive filter functionality. An example of an electronic duplex filter is described in U.S. Pat. No. 7,702,295, issued 20 Apr. 2010 to Nicholls et al., the entire contents of which are incorporated by reference herein to provide background for the disclosure that follows. In a TDD system, electronic cancellation circuits may assist with isolating the transmit signals from the sensitive receiver circuitry.